The present invention relates to a cellular radio system such as a car/portable telephone system and a cordless telephone system and, more particularly, to a cellular radio system using a code division multiple access (CDMA) scheme as a radio access scheme between a base station and a mobile station, and a base station apparatus and mobile station apparatus which are used for the cellular radio system.
A spread spectrum communication scheme, which is robust against interference and disturbance, has gained a great deal of attention as one of communication schemes used for a mobile communication system. The spread spectrum communication scheme is mainly used to implement a cellular radio system using CDMA scheme.
In the cellular radio system using CDMA scheme, for example, the transmitting apparatus modulates digital speech data or digital image data by a digital modulation scheme such as PSK. This modulated transmission data is then converted into a broadband baseband signal by using a spreading code such as a pseudorandom noise code (PN code). This baseband signal is up-converted into a signal in the radio frequency band and is then transmitted. The receiving apparatus down-converts the received signal in the radio frequency band into a signal having an intermediate or baseband frequency. The apparatus then de-spreads this down-converted signal by using the same spreading code as that used by the transmitting apparatus. Digital demodulation is then performed for the de-spread signal by a digital demodulation scheme such as PSK, thereby reconstructing data from the received data.
That is, in CDMA scheme, different spreading codes are assigned to radio communications between a plurality of mobile station apparatuses and base stations to ensure channel separation between the respective radio communications.
FIG. 14 shows the schematic arrangement of a CDMA cellular radio system. Referring to FIG. 14, a plurality of base stations BS1 to BSn are distributed in a service area. These base stations BS1 to BSn are respectively connected to a control station CS through wire lines L1 to Ln. The base stations BS1 to BSn are further connected to a wire network NW through the control station CS. The base stations BS1 to BSn respectively form radio zones Z1 to Zn called cells. Mobile stations MS1 to MSm are respectively connected to base stations BS in the cells where the respective mobile stations are present by CDMA scheme over radio paths.
In a system of this type, when any one of the mobile stations MS1 to MSm moves between cells during communication, handoff, i.e., the switching of the base station to another to which radio path is to be connected is performed. There are two types of handoff: soft handoff and hard handoff.
Soft handoff is unique to a CDMA cellular radio system. The perform handoff, a mobile station has two radio paths at the same time. One of the radio paths connects the mobile station to the source base station with which it has been communicating. The other radio path connects the mobile station to the destination base station with which it will communicate. The mobile station then performs path diversity synthesis, by using the signals it receives over these radio paths. Thereafter, of the paths under path diversity synthesis, a path on which the reception electric field strength of a pilot channel has dropped below a threshold for a predetermined period of time or more is disconnected, thereby switching the base stations to which the mobile station is connected. As described above, in a soft handoff, one of two paths is always connected to a base station at the time of handoff, and hence no path disconnection occurs. An advantage of soft handoff is that switching can be smoothly performed without any short break in speech.
Soft handoff, however, requires both the source and destination base stations to use the same radio frequency. For this reason, for example, as shown in FIG. 14, in a system in which different radio frequencies f1, f2, and f3 are allocated to a plurality of base station groups BSa, BSb, and BSc, when a mobile station MSi moves from a cell of the base station group Bsa to a cell of another base station group BSb or BSc, soft handoff cannot be performed.
In contrast to this, hard handoff is mainly performed when the above source and destination base stations use different frequencies. More specifically, when a mobile station must change the radio frequency in use at the time of handoff, a message for instructing handoff is sent from a base station to the mobile station. Upon reception of this message, the mobile station temporarily stops transmission/reception and forms a new radio path allocated by the base station between itself and the base station. After this radio path is formed, the mobile station resumes transmission/reception by using the path. In a hard handoff, the radio path is temporarily disconnected to switch the radio frequencies, and a radio path must be formed again by using a new radio frequency.
In such a system, a plurality of radio frequencies may be allocated to the respective base station groups, and the respective base stations may use CDMA scheme with the respective radio frequencies, thereby increasing the number of traffic channels.
In this case, however, the occupancies of the traffic channels having the respective radio frequencies in the respective base stations may become uneven. Assume that the occupancies of the traffic channels having the respective radio frequencies become uneven. In this case, if an originating mobile station selects a radio frequency with dense traffic, the base station may be found busy in spite of the presence of an available channel, resulting in blocked communication. Assume also that a mobile station selects a radio frequency with dense traffic in a hard or soft handoff. In this case, even if an available channel is present, it takes a long period of time to form a radio path again. As a result, speech communication may be interrupted or noise may be produced, deteriorating the speech communication quality. Alternatively, the call may be dropped due to a handoff failure.
It is an object of the present invention to provide a cellular radio system which can average the traffic channel occupancies of the respective radio frequencies, thereby allowing effective use of traffic channels.
In order to achieve the above object, according to the present invention, in a cellular radio system including a plurality of base stations respectively belonging to a plurality of base station groups and forming cells having a predetermined diameter, and mobile stations connected to the base stations over CDMA radio channels, each of the base stations using CDMA radio channels of a plurality of radio frequencies allocated to the base station group to which the base station belongs, the channel occupancies of a plurality of radio frequencies allocated to each of the plurality of base stations are monitored by a channel occupancy monitoring means.
When the difference between the channel occupancies of the respective radio frequencies allocated to the same base station, determined by the channel occupancy monitoring means, becomes a predetermined state, the radio frequency as a candidate to be used by a predetermined mobile station of mobile stations that are present in the cell formed by the base station and set in the standby state is switched to the radio frequency whose channel occupancy determined by the channel occupancy monitoring means is in a predetermined state.
Consequently, as radio frequencies to be used by mobile stations in the standby state to newly start communications, radio frequencies whose channel occupancies are in a predetermined state are properly distributed at random, thereby averaging the channel occupancies of the respective radio frequencies.
In addition, according to the present invention, a frequency use state notifying means generates frequency use state information for notifying the mobile station of the use state of each radio frequency on the basis of the channel occupancies determined by the channel occupancy monitoring means, and transmits the information to the mobile station. The mobile station forms a radio frequency list in which priority levels are assigned to a plurality of radio frequencies on the basis of the frequency use state information before power-off, and stores it in a power-off state. Immediately after power-on of the mobile station, a search is performed to check in the order of priority levels whether the radio frequencies indicated by the radio frequency list stored in the storage means can be used, and the first detected radio frequency that can be used is as a candidate radio frequency to be used.
Consequently, as radio frequencies to be used by mobile stations in the standby state to newly start communications, radio frequencies are properly distributed at random on the basis of the use state of each radio frequency before power-off, thereby averaging the channel occupancies of the respective radio frequencies.
When the mobile station moves to a new cell formed by another base station belonging to the same base station group to which the base station forming an old cell in which the mobile station has been present belongs, it is checked, on the basis of the channel occupancy determined by the channel occupancy monitoring means with respect to the base station forming the new cell, whether the channel occupancy of the radio frequency in the new cell, which has been used by the mobile station in the old cell, is not less than a predetermined value. Soft handoff is determined if the channel occupancy of the radio frequency in the new cell, which has been used by the mobile station in the old cell, is less than predetermined value. Hard handoff is determined if the channel occupancy of the radio frequency in the new cell, which has been used by the mobile station in the old cell, is not less than predetermined value. When hard handoff is to be performed, a radio frequency whose channel occupancy is not more than a predetermined value in the new cell is used in the mobile station.
Consequently, as radio frequencies to be used after mobile handoff, radio frequencies whose channel occupancies are not more than the predetermined value in new cells are properly distributed at random, thereby averaging the channel occupancies of the respective radio frequencies.